1. Field of the Invention
The present invention relates to a transmissive liquid crystal display apparatus with a wide viewing angle including a switching device such as a thin film transistor (hereinafter, referred to as a TFT).
2. Description of the Related Art
FIG. 9 is a circuit diagram showing a structure of a conventional transmissive liquid crystal display apparatus including an active matrix substrate.
Referring to FIG. 9, a plurality of pixel electrodes 1 are formed on the active matrix substrate in a matrix. TFTs 2 functioning as switching devices are connected to the pixel electrodes 1. Gate lines 3 (i.e., scanning lines for supplying scanning signals) are connected to gate electrodes of the TFTs 2. Gate signals input to the gate electrodes control so as to drive the TFTs 2. Source lines 4 (i.e., signal lines for supplying data signals) are connected to source electrodes of the TFTs 2, and data (display) signals are input to the pixel electrodes 1 via the TFTs 2 while the TFTs 2 are driven. The gate lines 3 and the source line 4 are running on the periphery of the pixels 1 arranged in a matrix and are orthogonally crossing each other. Furthermore, drain electrodes are connected to the pixel electrodes 1 and storage capacitors 5. Counter electrodes of the storage capacitors 5 are connected to common lines 6.
FIG. 10 is a cross-sectional view of the TFT portion of the active matrix substrate in the conventional liquid crystal display apparatus.
Referring to FIG. 10, the gate electrode 12 connected to the gate line 3 of FIG. 9 is formed on a transparent insulating substrate 11. A gate insulating film 13 is formed so as to cover the gate electrode 12. Furthermore, a semiconductor layer 14 is formed on the gate insulating film 13 so as to overlap the gate electrode 12. A channel protective layer 15 is formed in the central portion of the semiconductor layer 14. An n.sup.+ Si layer to function as a source electrode 16a and a drain electrode 16b is formed so as to cover both end portions of the channel protective layer 15 and a part of the semiconductor layer 14 and to be divided by the channel protective layer 15. A metal layer 17a to function as a source line 4 of FIG. 9 is formed on one of the n.sup.+ Si layers, i.e., the source electrode 16a, while a metal layer 17b for connecting the drain electrode 16b and the pixel 1 is formed on the other n.sup.+ Si layer, i.e., the drain electrode 16b. Furthermore, an interlayer insulating film 18 is formed so as to cover the TFT 2, the gate line 3 and the source line 4.
A transparent conductive film to function as the pixel electrode 1 is formed on the interlayer insulating film 18. The transparent conductive film is connected to the metal layer 17b connected to the drain electrode 16b of the TFT 2 via a contact hole 19 penetrating the interlayer insulating film 18.
As described above, the interlayer insulating film 18 is formed between the gate line 3 and the source line 4 and the transparent conductive film to function as the pixel electrode 1. For this reason, the pixel electrode 1 can overlap the gate and source lines 3 and 4.
Conventionally, an inorganic film such as silicon nitride (SiN) of a thickness of about 500 nm is formed by a CVD method for the interlayer insulating film 18. Therefore, the pixel electrode 1 is generally formed with a step d of about 1 .mu.m or more due to the underlying structure.
Furthermore, in such a conventional liquid crystal display apparatus, the maximum viewing angle is about .+-.50.degree. to the right and left directions. As techniques for enlarging the viewing angle, a technique of dividing the pixel into multi-domains (pixel divisional alignment) or various other techniques are disclosed, for example in "Flat Panel Display, 1994" (pp.166) by Nikkei BP, Japanese Laid-Open Patent Publication No. 7-36043 or the like.
Furthermore, in recent years, a large TV screen has been developed. In addition, a wider screen (i.e., horizontally wide TV screen) has been increasingly the trend. In a liquid crystal TV set, a product with a wide screen has been researched and developed.
FIG. 11 is a view quantatively showing the relationship between a conventional TV screen 120 with the ratio of the shorter side to the longer side of 3:4 and a wide screen 121 with the ratio of the shorter side to the longer side of 9:16. In the case where a viewer views the wide screen 121 at an angle, in order for the viewer to view the screen end A on the side farther from the viewer, the screen is required to have a wider viewing angle .theta..sub.1 than the viewing angle .theta..sub.2 of the conventional TV screen 120 (i.e., .theta..sub.1 &gt;.theta..sub.2) . Herein, the viewing angle refers to a gradient angle with respect to the normal direction of the screen (i.e., substrate).
In the conventional liquid crystal display apparatus as described above, when the interlayer insulating film 18 is formed between the gate line 3 and source line 4 and the pixel electrode 1, the pixel electrode 1 can be formed so as to overlap the gate and source lines 3 and 4, thus improving an aperture ratio of the liquid crystal display apparatus and restricting unsatisfactory alignment of liquid crystal molecules. However, in the case where the pixel electrode 1 is overlapping the gate and source lines 3 and 4, a problem of an increase in capacitance between the gate and source lines 3 and 4 and the pixel electrode 1 is caused. In particular, an inorganic film such as silicon nitride film has a dielectric constant as high as 8, and is formed by a CVD method. As a result, the film has a thickness of about 500 nm. With such a thickness, the capacitance between the gate and source lines 3 and 4 and the pixel electrode 1 is increased, thus causing the following problems (1), (2) and (3). It is difficult to mass-produce an inorganic film such as silicon nitride film of a thickness of more than about 500 nm.
(1) In the case where the pixel electrode 1 overlaps the source line 4, the capacitance between the source line 4 and the pixel electrode 1 is increased, thereby raising a ratio of signal transmission. As a result, data signals stored in the pixel electrode 1 during a storage period are subjected to oscillation due to electric potentials of the data signals. Therefore, an effective voltage applied to the liquid crystal of the pixel fluctuates, so that vertical crosstalk in the vertical direction occurs, in particular, with respect to the adjacent pixel in the vertical direction in an actual display.
(2) In the case where the pixel electrode 1 overlaps the gate line 3 for driving the pixel, the capacitance between the gate line 3 and the pixel electrode 1 increases, thus increasing feed through of a voltage for writing in the pixel due to a switching signal for controlling the TFT 2.
(3) The dielectric constant of the interlayer insulating film 18 is as high as 8, and affected by the potential of the pixel electrode 1 or the counter electrode (not shown) formed on the counter substrate. In addition, the interlayer insulating film 18 is polarized by an electric field in the vicinity of the TFT in reliability tests such as light aging or the like. As a result, the switching characteristics of the TFT deteriorates, thus causing a problem in reliability such as unsatisfactory display.
Furthermore, in the vicinity of the step portion d, an alignment film (not shown) is not properly rubbed due to the surface state. Moreover, the interlayer insulating film 18 is relatively thin, and the dielectric constant is high. These two facts synergistically serve to generate a strong electric field, especially in the direction from the gate electrode 12 to the surface of the substrate so as to disturb a desired alignment of liquid crystal molecules on the periphery of the pixel.
Furthermore, in the vicinity of the contact hole 19, the alignment of the liquid crystal molecules is easily disturbed for a similar reason as described above resulted from the generation of a step. Therefore, a light-shielding film (not shown) for covering the unsatisfactory display portion is provided on the same substrate or the counter substrate (not shown). However, the provision of the light-shielding film lowers an aperture ratio, resulting in a dark screen. Thus, the viewing angle is limited because of the low aperture ratio and the provision of an unnecessary light-shielding film.
Furthermore, although the techniques such as a multi-domain system in order to improve the viewing angle are disclosed in the Publications described above or the like, they have the following problems:
1. Since the number of steps and the number of types of materials increases and the yield deteriorates, the cost increases. PA1 2. Since delicate divisional alignment control is performed in micro regions, reliability is generally low. PA1 3. Since light is dispersed due to divisional alignment or the like, luminance is lowered in a certain direction or the like, thus leading to a low contrast ratio. Namely, visibility deteriorates and the viewing angle is restricted.
As a result, the maximum viewing angle to the right and left directions is about 50.degree. in the current mass production level or development level. Thus, a TFT liquid crystal display apparatus having a wide viewing angle and an excellent visibility and capable of being produced at a low cost has not been mass-produced yet.
Furthermore, as shown in FIG. 11, the wide screen type liquid crystal TV has a low visibility at the end of the screen, thus being unable to provide a satisfactory display to a large number of people in a large area.